1. Field of the Invention
The following disclosure relates generally to an image forming apparatus configured to re-use toner particles.
2. Discussion of the Background
Conventionally, a monochrome image forming apparatus such as monochrome copier and monochrome printer is configured to collect toner particles remaining on a photoconductive drum (i.e., image carrying member) after a transfer process with a cleaning unit, and to supply the collected toner to a developing unit to re-use the collected toner particles in view of resource saving and longer lifetime of the image forming apparatus. Some background apparatuses use a method that a cleaning unit collects not-transferred toner particles by a cleaning unit, and such not-transferred toner particles are conveyed to a toner refining unit, then separated by a magnet roller in the toner refining unit, and conveyed to a developing unit. Other background apparatuses use a method in which a two-component developer stored in a developing unit is circulated between the developing unit and a cleaning unit. Toner particles collected in the cleaning unit are mixed with the circulating developer, and used as “recycled toner.”
As for color image forming apparatuses, such as color copiers and color printers, there is a need for apparatuses that can stably produce high quality images even if some environmental conditions are changed. Such color image forming apparatuses have employed a two-component developing method using a two-component developer having non-magnetic toner particles and magnetic carrier particles.
As for the two-component developing method, a developing roller (i.e., developer carrier) having a magnet inside the developing roller is applied with a predetermined developing bias voltage. At this time, the magnetic carrier particles aggregate on the developing roller along magnetic field lines formed around the magnet to form a magnetic brush. The non-magnetic toner particles adhere to the magnetic brush. With such an arrangement, the two-component developer is carried on the developing roller, and the non-magnetic toner particles in the two-component developer are transferred and adhered on an electrostatic latent image formed on a photoconductive drum.
The above-mentioned background apparatuses experience image quality degradation such as background fogging and degraded granular quality over time. Therefore, using the toner particles collected by the above-mentioned background apparatuses as “recycled toner” is unfavorable for the color image forming apparatus in view of stably producing high quality images. Specifically, the two-component developer used for color image forming apparatuses typically includes additives on a surface of toner particles such as silica and titanium oxide in order to improve disperseability of toner particles. Theses additives are susceptible to mechanical stress and heat stress, thereby the additives may be buried inside the toner particles, or may be dropped off from the surface of toner particles when an agitator agitates the developer in the developing unit, which can result in a change of properties (e.g., fluidity) of the developer. Accordingly, the amount of developer to be carried-up to a developing area of the developing roller may be reduced, thereby causing image quality degradation such as lower granular quality.
Such a drawback can be observed when the collected toner particles are used as “recycled toner.” Specifically, the collected toner particles, which are collected in the cleaning unit, may receive stresses such as mechanical stress and heat stress when the collected toner particles are conveyed to the developing unit from the cleaning unit, whereby the additives on the toner particles may be buried inside the toner particles, or may be dropped off from the surface of toner particles.
Furthermore, the collected toner particles in the cleaning unit may have unstable charge properties (e.g., charge rising property) due to a transfer bias voltage applied during a transfer process. Such toner particles may not regain stable charge properties even if such toner particles are mixed with the developer in the developing unit. When such toner particles are used as “recycled toner,” such toner particles may not adhere to carrier particles properly, thereby the toner particles may spatter in the developing unit, or may adhere on a non-image area of the photoconductive drum and form a background fogging in a printed image.
Such drawbacks may become more obvious if the carrier particles in the developing unit are used for a longer time and lower its charge-ability (hereinafter referred as “CA”) for the toner particles.
The above-mentioned background apparatuses use methods that collect toner particles in the cleaning unit and convey the collected toner particles to the developing unit via the toner refining unit. Accordingly, when such collected toner particles are conveyed in a conveying line, these toner particles receive stresses such as direct mechanical stress from conveying unit members and from collisions with other toner particles. In such conditions, the additives may be buried in the toner particles or dropped from the toner particles, and toner particles may aggregate, or change their charge property. As a result, image quality degradation such as toner particles spattering, background fogging, and lower granular quality may happen. Such drawbacks may become further obvious if the carrier particles in the developing unit degrade over time.
On one hand, such a background apparatus circulates the two-component developer stored in the developing unit between the developing unit and the cleaning unit, and mixes toner particles collected in the cleaning unit with the circulating developer. Therefore, when the collected toner particles are conveyed in a circulating line, the collected toner particles receive a lower stress such as direct mechanical stress from members composing a conveying unit and from collisions with other toner particles. However, if the carrier particles in the circulating developer degrade over time, image quality degradation such as toner particles spattering, background fogging, and lower granular quality may happen. In addition, the amount of impurities such as dropped additives and paper powders may increase in the circulating line over time, whereby such impurities may affect the properties of the developer, and result in an unstable developing process.